From South Carolina to Texas, salt marshes have experienced a massive die-off in recent years, threatening fisheries and leaving coastal areas vulnerable to flooding. The culprit, ecologists have long thought, is degraded soil. But new research, published in Science, points to the periwinkle – cordgrass consuming sea snails – as a major contributor to salt marsh loss.

PROVIDENCE, R.I. — Periwinkles, the spiral-shelled
snails commonly found along rocky U.S. shorelines, play a primary role in the
unprecedented disappearance of salt marsh in the southeastern states, according
to new research published in Science.

Based on extensive field studies, the work challenges six
decades of salt marsh science. Ecologists have long thought that stressed soil
– too much salt, not enough oxygen – was the main killer of this
critical marine habitat.

Salt marsh assassin
Periwinkles, a saltwater snail that is ubiquitous along the U.S. shore, can reduce healthy salt marshes to mud flats in a matter of months. Ecologists had long thought salt-marsh die-offs were related to degraded soil.

But Brian Silliman, a Brown University research fellow and a
University of Florida assistant professor, said drought-stressed soils pave the
way for predatory periwinkles that spread fungal disease as they graze on
cordgrass.

“Snails can transform healthy marsh to mudflats in a
matter of months,” said Silliman, lead author of the Science paper.
“This finding represents a huge shift in the way we see salt marsh
ecology. For years, scientists thought marsh die-off was simply a
‘bottom-up’ problem related solely to soil conditions. We found that
the trouble also comes from the top down. Drought makes the marsh vulnerable,
then the snails move in.”

Thousands of acres of salt marsh have disappeared from South
Carolina to Texas since 2000, according to several scientific studies. In
Louisiana alone, more than 100,000 acres of marsh were severely damaged between
June 2000 and September 2001. This drastic decline poses a serious threat to the
ecology and economy of the southeastern seaboard and the Gulf Coast. Salt
marshes serve as nursery grounds that support commercial fisheries, protect
coastline from storm-induced floods, and filter fresh water before it flows out
to sea.

Mark Bertness, chair of the Department of Ecology and
Evolutionary Biology at Brown and a co-author of the paper, said a better
understanding of the causes of salt marsh loss will point to better ways to
protect them.

“Loss of blue crabs and turtles, which prey on
periwinkles, allows the snails to flourish,” Bertness said. “Protect
the crabs and turtles and you can help save the marshes.”

Silliman came up with the periwinkle premise as a graduate
student conducting field research in Virginia. Silliman found that removing
snails from cordgrass, the dominant plant species in salt marshes, bumped up
grass growth as much as 50 percent.

Silliman earned his Ph.D. at Brown and worked in the Bertness
lab along with Johan van de Koppel, a former postdoctoral research associate now
at the Netherlands Institute of Ecology. For more than two years, the trio
tested Silliman’s top-down theory of marsh ecology along the Georgia,
South Carolina and Louisiana coasts in conjunction with Lousiana State
University researchers Lee Stanton and Irving Mendelssohn.

In 12 randomly selected die-off sites, the team surveyed
periwinkle populations. They found the largest concentration of snails –
as many as 2,000 per square meter – along dead-zone borders. To test the
idea that the snails contribute to cordgrass death, they created dozens of
deterrents – wire mesh enclosures measuring about one meter square.
Enclosures were placed ahead of fronts of grass-grazing snails and monitored for
more than a year.

The results: Inside the enclosures, snail-free cordgrass
thrived. In fact, plant biomass increased more than threefold. Outside the
cages, in 11 of the 12 sites, snail overgrazing converted healthy marsh to
exposed mudflats in as little as three months. When snail density was high,
destruction was more extensive.

Researchers also wanted to test the notion that increased soil
salinity, brought on by drought, acts in concert with snails to kill marshland.
So in one healthy site in Georgia, the team elevated soil salt concentrations in
areas with and without snails. Sites were monitored for eight months.

In the experimental plots, increased salinity reduced grass
growth by 45 percent while high salt levels, in combination with the presence of
snails, reduced grass growth by 84 percent.

How do periwinkles contribute to marsh destruction? Silliman has
shown that they kill the grass by slicing the stems during grazing, leaving
plants vulnerable to harmful fungi. In a process called “fungal
farming,” snails then consume this fungi living off injured grass.

“We’ve found a synergism between climate change and
grazers,” Silliman explained. “Severe drought triggers formation of
traveling fronts of grazing snails. Then there is runaway consumption, which
leads to waves of marsh destruction. Given predicted increases in climate
change-induced drought, these results highlight the potential for marsh die-off
to be even more intense and extensive in the future.”

The findings, the authors argue, underscore the interplay of
climate and consumers in the worldwide collapse of coastal systems. While an
overabundance of snails may fuel southeastern salt marsh destruction, they point
to other examples of habitat destruction that may be caused, in part, by a
plethora of grazers: sea urchins wiping out California kelp beds, sea stars
devastating Australian coral reefs, snow geese decimating marshes along the
Artic Sea, bark beetles killing off Arizona pine forests.

Georgia Sea Grant, Louisiana Sea Grant, The Nature Conservancy,
the National Science Foundation and the Schure-Beijerinck-Popping Fund supported
the work.